This invention relates to a semiconductor dynamic random access memory (“DRAM”) cell, array and/or device and method of controlling and/or operating a semiconductor memory cell array and/or device; and more particularly, in one aspect, to a semiconductor dynamic random access memory (“DRAM”) cell, array and/or device wherein the memory cell includes an electrically floating body in which an electrical charge is stored.
There are many different types and/or forms of DRAM cells, including, for example, a semiconductor memory cell consisting of an access transistor and a capacitor, which stores an electric charge representing a bi-stable memory state. The access transistor serves as a switch for controlling the charging and discharging of the capacitor as well as reading and writing of the logic states into the capacitor (i.e., charging or discharging the capacitor).
Although significant integration densities can be achieved with DRAM devices employing one transistor-one capacitor memory cells, such devices tend to be limited or restricted with respect to the size of the memory cell. In this regard, conventional techniques employ stacked and/or trench capacitor approaches, whereby the capacitor is partially disposed above and/or below an access transistor.
In addition, DRAM devices employing one transistor-one capacitor memory cells tend to be fabricated using manufacturing processes that are different from and/or incompatible with manufacturing processes for logic devices (for example, microprocessors). As a result, integration of one transistor-one capacitor memory cells into logic devices is often complicated and expensive.
Another type of dynamic random access memory cell is described and illustrated in non-provisional patent application entitled “Semiconductor Device”, which was filed on Jun. 10, 2003, and assigned Ser. No. 10/450,238 (hereinafter “Semiconductor Memory Device Patent Application”). With reference to FIGS. 1A and 1B, the Semiconductor Memory Device Patent Application discloses, among other things, semiconductor DRAM device 10 in which each memory cell 12 consists of transistor 14 having gate 16, body region 18, which is electrically floating, source region 20 and drain region 22. The body region 18 is disposed between and adjacent to source region 20 and drain region 22. Data is written into or read from a selected memory cell by applying suitable control signals to a selected word line(s) 24, a selected source line(s) 26 and/or a selected bit line(s) 28. In response, charge carriers are accumulated in or emitted and/or ejected from electrically floating body region 18 wherein the data states are defined by the amount of carriers within electrically floating body region 18.
In particular, in one embodiment, the memory cell of the Semiconductor Memory Device Patent Application operates by accumulating in or emitting/ejecting majority carriers (electrons or holes) 30 from body region 18 of N-channel transistors. (See, FIGS. 2A and 2B). In this regard, accumulating majority carriers (in this example, “holes”) 30 in body region 18 of memory cells 12 via, for example, impact ionization near source region 20 and/or drain region 22, is representative of a logic high or “1” data state. (See, FIG. 2A). Emitting or ejecting majority carriers 30 from body region 18 via, for example, forward biasing the source/body junction and/or the drain/body junction, is representative of a logic low or “0”. (See, FIG. 2B).
Various techniques may be employed to read the data stored in (or write the data into) a memory device of the Semiconductor Memory Device Patent Application. For example, a current sense amplifier (not illustrated) may be employed to read the data stored in memory cells 12. In this regard, a current sense amplifier may compare the memory cell current to a reference current, for example, the current of a reference cell (not illustrated). From that comparison, it may be determined whether memory cell 12 contained a logic high (relatively more majority carriers 30 contained within body region 18) or logic low data state (relatively less majority carriers 28 contained within body region 18).
Notably, transistor 14 may be a symmetrical or non-symmetrical device. Where transistor 14 is symmetrical, the source and drain regions are essentially interchangeable. However, where transistor 14 is a non-symmetrical device, the source or drain regions of transistor 14 have different electrical, physical, doping concentration and/or doping profile characteristics. As such, the source or drain regions of a non-symmetrical device are typically not interchangeable.
The transistor 14 may be controlled using a negative drain voltage on, for example, bit line 28i, to remove holes from electrically floating body region 18 through drain 22 to write a logic low (i.e., binary state “0”). Under this circumstances, a negative voltage applied to gate 16 of the other (non-selected) memory cells in the memory array of device 10 may be necessary to avoid “leakage current” in other cells connected to the same bit line 28i when the negative bit line voltage is applied during the write (logic low) operation.
Other operations such as writing a logic high data state (binary “1”) and reading the data may be performed using positive voltages applied to word lines 24. As such, transistors 14 of device 10 are periodically pulsed between a positive gate bias, which (1) drives majority carriers (holes for N-channel transistors) away from the interface between gate insulator 32 and body region 18 of transistor 14 and (2) causes minority carriers (electrons for N-channel transistors) to flow from source region 20 and drain region 22 into a channel formed below gate 16, and a negative gate bias, which causes majority carriers (holes for N-channel transistors) to accumulate in or near the interface between gate 16 and body region 18 of transistor 14.
With reference to FIG. 3A, a positive voltage applied to gate 16 provides a positive gate bias which causes (1) a channel of minority carriers 34 to form beneath gate 16 and (2) accumulation of majority carriers 30 in body region 18 in an area “opposite” the interface of gate 16 and body region 18. Here, minority carriers (i.e., electrons in an N-channel transistor) may flow in the channel beneath the interface of gate oxide 32 and floating body region 18 wherein some of the minority carriers 34 are “trapped” by or in defects within the semiconductor (typically created or caused by the transition from one material type to another).
With reference to FIG. 3B, when a negative voltage is applied to gate 16, the gate bias is negative which substantially eliminates the channel of minority carriers 34 beneath gate 16 (and gate oxide 34). However, some of minority carriers may remain “trapped” in the interface defects (illustrated generally by electrons 36).
Some of the trapped electrons 36 recombine with majority carriers which are attracted to gate 16 (due to the negative gate bias), and, as such, the net charge of majority carriers 30 located in floating body region 18 may decrease over time (see, for example, FIG. 3C). This phenomenon may be characterized as charge pumping. Thus, pulsing between positive and negative gate biases (during read and write operations) may reduce the net quantity of charge in memory cell 12, which, in turn, may gradually eliminate the data stored in memory cell 12.
Notably, for the efficient charge pumping phenomenon to occur, the free electron concentration at the surface (ne) in inversion should be sufficiently large that the interface traps can capture electrons during the time the transistor is in inversion. The time constant for electron capture may be characterized as:
      τ    e    =      1                  v        th            ·              σ        n            ·              n        e            
Accordingly, in the case τe=3 ns (typical pulse duration in advanced DRAM memories), the thermal velocity vth=1×107 cm/s and the capture cross section σn=2×10−16 cm2, at least ne≈2×1017 cm−3 may be required. Analogously, in accumulation the free hole concentration at the surface (nh) should be sufficiently large that holes can recombine with the captured electrons during the time transistor 14 is in accumulation. If σn=σp, nh2≈1017 cm−3 at least may be required (i.e., the efficient charge pumping effect exists if the gate voltage in inversion accumulates at least 2×1017 cm−3 electrons at the surface and the gate voltage in accumulation accumulates at least 2×1017 cm−3 holes).
Notably, the entire contents of the Semiconductor Memory Device Patent Application, including, for example, the features, attributes, architectures, configurations, materials, techniques and advantages described and illustrated therein, are incorporated by reference herein.